Experts have criticised plans to subsidise low-carbon heat pumps in place of gas boilers for homeowners in England and Wales from next April.
Households will be offered subsidies of £5,000 from next April to help them make the switch.
Although up to 25 million UK homes have gas boilers, the grants will fund just 90,000 pumps over three years.
The government says this will boost demand for the pumps, but critics say the plan does not go far enough.
Homeowners will be encouraged to switch to a heat pump or other low-carbon technology when their current boiler needs replacing.
An air-source heat pump costs between £6,000 and £18,000, depending on the type installed and the size of a property.
Ministers say the subsidies will make heat pumps a comparable price to a new gas boiler. However, the £450m being allocated for the subsidies over three years has been criticised as insufficient.
Experts also point to the need for costly new insulation and other home improvements to help households get the best out of the switch.
“Gas is in the news for price increases that are on the horizon,” said Dr David Glew, head of energy efficiency and policy at the Leeds Sustainability Institute at Leeds Beckett University.
“But gas is actually relatively cheap to heat your home with,” he added.
“Needing to insulate your house might cost you tens of thousands of pounds and you’re only going to be saving several hundreds of pounds, so the economics of that doesn’t really add up.”
Mike Childs, head of science at Friends of the Earth, said the number of heat pumps that the grants would cover “just isn’t very much” and meant the UK would not meet its climate targets.
“These grants will only incentivise the best-off households,” he said.
Greenpeace UK’s climate campaigner, Caroline Jones, said the government needed to provide more money to speed up the switch.
“A clearer signal would have been a phase-out of new boilers before 2035,” she said.
The government says it wants no new gas boilers to be sold after 2035, but it has not banned them outright.
Writing in the Sun, Prime Minister Boris Johnson said: “The Greenshirts of the Boiler Police are not going to kick in your door with their sandal-clad feet and seize, at carrot-point, your trusty old combi.”
Downing Street later declined to say whether the government would ban gas boilers in the future, after international trade secretary Anne-Marie Trevelyan said “there will be a point” when the voluntary approach would change.
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Jonny Marshall, senior economist the Resolution Foundation, a think tank focusing on poverty, said the UK would struggle to meet its goal of cutting emissions from homes in half by 2035.
The UK has committed to reduce its greenhouse gas emissions to net zero by 2050.
But a group of experts that advises the government says Boris Johnson’s government has credible policies in place to deliver only about a fifth of this cut.